In an image forming method by an electro photographic process, an image is generally formed by the following steps: forming an electrostatic latent image on an electrostatic latent image carrier; developing the electrostatic latent image with a toner to obtain a toner image; transferring this toner image to an intermediate transfer member; then again transferring the transferred toner on the intermediate transfer member to an image forming support, such as paper; and fixing the image on the image forming support to form a final fixed toner image (for example, refer to Patent Document 1).
In an image forming method of a digital mode, in order to from of an electrostatic latent image, light exposure is performed, for example, by pulse modulated dots, and reversal development is carried out by allowing the toner to adhere on the portion applied the dots (the dot portion). The amount of the toner adhering to the dot portion is determined according to the area of the dot portion on an electrostatic latent image carrier, the state of an electric potential on an electrostatic latent image carrier, the developing bias to the dot portion and the state of the electric charge possessed by the toner. A high density image portion which is recorded on an electrostatic latent image carrier by a light exposure device has a large number of dots per unit area and the size of one dot is large. On the other hand, A low density image portion has a small number of dots per unit area and the size of one dot is small. As a result, a high density image portion on an electrostatic latent image carrier has a large toner adhesion amount after development, while a low density image portion has a small toner adhesion amount. Thus, the difference between a high and a low density image is expressed by the toner adhesion amount per unit area which adhered on an electrostatic latent image carrier. By this reason, there is a problem that a low density image portion (it is also referred to as “a highlight portion”) tends to have a deteriorated granularity, and as a consequence, there is a problem that a stable gradation is hard to obtain. Here, “deteriorated granularity” indicates the state that gives a feeling of detailed roughness of light and shade and an inhomogeneous feeling which are visually recognized in the highlight portion homogeneously exposed.
In order to resolve such a problem, there is disclosed a technology which enables to perform an image formation excellent in granularity even in a highlight portion by using a gray toner (for example, refer to the Patent Document 2 and Patent Document 3.)
However, even when a gray toner is used, since glossiness of an image formed by an electrophotographic process is controlled by the toner image composed mainly of a binder resin and a colorant and also by the surface smoothness of the wax layer which bled out on the toner image, a high density image portion having a high toner adhesion amount (it is also referred to “a shadow portion”) tends to have a high glossiness, while a highlight portion having a remarkably small toner adhesion amount tends to have a glossiness which depends on the image forming support background. The difference of glossiness between a shadow portion and a highlight portion has been an obstacle to improve the image quality.
On the other hand, to the photographic image produced by a color image forming method, the colorfulness of color is required and development of new coloring materials for a color toner is progressing (for example, refer to Patent Document 4 and Patent Documents 5.) In these developments, although it became possible to increase chroma and lightness of a color toner, there occurred the following problem.
The problem is a difficult reproduction of a soft tone image and a soft tone image. A soft tone and a soft tone are a mixed color located between a vivid tone (pure color) and a dark grayish tone (shade, a color obtained by adding black to a pure color). A vivid tone has a high chroma and a high lightness, one the other hand a dark grayish tone has a low chroma and a low lightness. A light tone (tint, a color obtained by adding white to a pure color) is produced by decreasing an image area ratio of a vivid tone. And a soft tone is a color produce by adding a small amount of darkness to a light tone. A dull tone is a color by adding a more amount of darkness to a light tone than a soft tone. In addition, a dark tone is a color produced by further adding darkness to a dull tone.
“A soft tone” gives a feeling of soft and familiar, therefore it is used for a wide range of application. “A dull tone” gives a feeling of heavy and quiet, therefore it is used for a fashion or an interior color.
The catalogue which shows product images are required a faithful color reproduction, because, a subtle difference of color between the print and the real object will change the total impression. Therefore, it is required a specific toner which enables to control the chroma and lightness of a color toner.
Examples of “a soft tone” expressed in color code used to indicate colors in a Web page include: #e09696, #e0ce96, #bbe096, #96e0a9, #96e0e0, #96a9e0, #bb96e0, #e096ce, #cc7a7a, #cc967a, #ccb17a, #cccc7a, #b1cc7a, #96cc7a, #7acc7a, #7acc96, #7accb1, #7acccc, #7ab1cc, #7a96cc, #7a7acc, #967acc, #b17acc, #cc7acc, #cc7ab1 and #cc7a96.
Examples of “a dull tone” expressed in color code include: #b35959, #b37759, #b39559, #b2b359, #95b359, #77b359, #59b359, #59b377, #59b395, #59b3b3, #5995b3, #5977b3, #5959b3, #7759b3, #9559b3, #b259b3, #b35995, #b35977, #8c3f3f, #8c663f, #8c8c3f, #668c3f, #3f8c3f, #3f8c66, #3f8c8c, #3f668c, #3f3f8c, #663f8c, #8c3f8c, #8c3f66, #996666, #999966, #669966, #669999, #666699 and #996699.    Patent Document 1: U.S. Pat. No. 2,297,691    Patent Document 2: Japanese Patent Application Publication (JP-A) No. 2004-133247    Patent Document 3: JP-A No. 2006-227308    Patent Document 4: JP-A No. 2007-140478    Patent Document 5: JP-A No. 2007-34264